Optical resonators are known, for example, from the publication “Optische Resonatoren für Hochleistungs-Festkörper-Laser” (Optical Resonators for high-power solid-state Lasers) by N. Hodgson, Festkörper-Laser-Institut Berlin GmbH, internal report, December 1990 or from the article “Analysis of stable-unstable free electron laser resonators” by C.-C. Shih, SPIE Vol. 1868, pages 278 to 285. The article “Improvement of slab-laser beam divergence by using an off-axis unstable-stable resonator” by K. Kuba et al. (Optics Letters, Vol. 15, 1990, pages 121 to 123) also discloses resonators. In particular, cylindrical mirrors are used in the optical resonators disclosed in these references.